The invention is concerned with the injection moulding of thermoplastics materials, in which the plastics material is plasticized by a reciprocating screw plasticizer which then acts as an injection ram to inject the material through a heated runner system into a mould. In particular, the invention is concerned with the problems which arise when the components to be moulded are large in size.
With large moulds it is often necessary to maintain the pressure of injection for a considerable time after completion of the injection stroke due to both the longer time required for the molten thermoplastics material, or melt, to fill the cavities of the mould and the inevitable shrinkage which takes place during cooling. When the shape of the mould cavity is intricate, and as a result of the inevitable shrinkage, sink marks or weak spots often occur in the moulded component. By maintaining the injection pressure such defects can be minimized. However, if the screw injector is used to prolong the pressure, the injection cycle has to be lengthened because otherwise there may not be sufficient time for plasticization of the next shot. This, of course, results in an inefficient and thus more costly process.
In order to overcome this problem and in accordance with the present invention, in a method of injection moulding, after injection of a shot of fluent thermoplastics material by a screw injector through a runner system into a mould, a positive pressure is maintained on the runner system while the injection screw returns for plasticization of the succeeding shot of plastics material.
Preferably, after the injection stroke a valve is closed between the injection nozzle and the runner system to prevent the positive pressure applied to the runner system from also effecting the plasticizing unit.
By this method, a pressure slightly less than the standard injection pressure may be maintained in the runner system and hence in the mould cavity completely independently of the back-pressure requirements of the screw during the plasticizing process.
The invention also includes apparatus comprising an injection mould having a mould runner system and means for maintaining a positive pressure on the mould runner system.
In order to maintain the required pressure a small reservoir for the fluent plastics material may be connected to the runner system, the reservoir being partly or wholly filled during the injection stroke, and this small amount of material thereafter being forced back into the runner system under pressure. The reservoir may be provided, for example, by a cylinder in which a piston is arranged to slide, the piston being in connection on one side, with the runner system and being acted upon from the other side by a pressure source such as a coil spring or hydraulic or pneumatic pressure.
As an example, the invention may be carried out by means of an apparatus in which the injection nozzle is extended in the form of a cylindrical piston clamped to the end of the injection unit. Surrounding the piston is a nozzle sleeve which may be mounted on the mould itself. The sleeve has a cylindrical recess in which the piston slides and the recess has a frustoconical end cooperating with the end of the piston nozzle. The piston nozzle is slidable in the sleeve to allow the formation of a reservoir of plastics material on injection so that, by means of a pull-in cylinder, material may be injected into the mould after the injection shot in order to prevent sink marks or wheel spots occuring in the finished moulded article.
This apparatus can also be used in accordance with the invention to increase the capacity of each shot of thermoplastics material into the mould. Thus, it can provide a method of injection moulding in which a plasticized thermoplastics material is injected from a reciprocating screw plasticizing unit into a mould, and in which a positive pressure is maintained on the runner system while the injection screw returns for plasticization of the succeeding shot of plastics material, and, as a preliminary step, a reservoir positioned in series between the plasticizing unit and the mould is filled with plasticized thermoplastics material prior to the injection stroke and, on injection, the material in the reservoir is injected into the mould.
By prefilling the reservoir with plasticized thermoplastic material an additional volume of material equivalent to the volume of the reservoir can be injected into the mould. Thus, the volume of injected thermoplastics material, and hence the size of the moulding, can be increased without having to increase the size of the injection-plasticizing unit. Preferably, the reservoir is formed between the injection nozzle and the mould cavity.
Objects and advantages of the invention will become apparent to one skilled in the art to which the invention pertains from the following detailed description read in conjunction with the appended drawings.